To protect children against whooping cough, doctors recommend five shots of vaccine before they turn 7.

But what happens after that? How long does the protection last?

Researchers at the Kaiser Permanente Vaccine Study Center checked it out. They compared 277 kids who came down with whooping cough, or pertussis, to more than 3,000 children who tested negative for it. Another comparison group consisted of more than 6,000 kids who were close matches to the sick kids on a bunch of factors, including sex, age and race.

The key finding: The protection from the vaccine drops by 42 percent each year after the fifth shot. So kids who got all their shots on time could become quite susceptible to whooping cough again between ages 8 and 11.

The researchers concluded we need a better vaccine than the current DTaP shot. (DTaP stands for diptheria, tetanus and acellular pertussis.) "Prevention of future outbreaks will be best achieved by developing new pertussis-containing vaccines that provide long-lasting immunity," they wrote.

The pertussis vaccine used today is relatively new. It's made with purified parts of the pertussis bacteria that spur an immune response, rather than whole pertussis cells that were in the standard vaccine until the early 1990s. The switch was made because the older vaccine had more side effects, such as swelling and pain where it was injected.

The new vaccine works, but not quite as well as was thought. "We know the short-term protection is very good," Tom Clark, an epidemiologist with the Centers for Disease Control and Prevention toldUSA Today. "But the protection is wearing off and that is the problem."

Other researchers have zeroed in on the same issue. Last month scientists reported that children in Australia were about four times more likely to catch whooping cough after receiving the DTaP vaccine than those who got the older kind.

Spotty vaccination rates are a problem in some places, like California, where whooping cough has roared backed recently. But the weakness of the current vaccine appears to be another factor in the resurgence of whooping cough.

Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.